A redox flow secondary battery includes porous electrodes (anode and cathode) on both sides of an ion-exchange membrane (diaphragm), a bipolar plate, and a frame. The bipolar plate serves to separate cells of a stack. In the bipolar plate, conductivity is required in order to minimize the internal resistance of the battery, and an electrolyte solution must not leak to the adjacent cell, but must be completely prevented from leaking. Further, the bipolar plate is required to have high mechanical strength (tensile strength) so as to prevent thermal shrinkage in response to a pressure and temperature change caused by the electrolyte solution, and is also required to be drawable so as to prevent breakage due to slight deformation.
A conventional bipolar plate includes a conductive filler (carbon material) or structure (carbon bar), a binder resin (thermoplastic or thermosetting), and a functional additive. Conventionally, in manufacturing a separator plate or a bipolar plate of a redox flow battery (hereinafter, referred to as a ‘bipolar plate’), a graphite bar is cut to a predetermined size using cutting and roughing, surface grinding is performed, a resin impregnation process is repeated 3 times or more, and precision cutting and surface grinding processes are performed to thus obtain a final product meeting a desired standard. However, the conventional process is characterized in that the cost of manufacturing the product is very high and in that the uniformity of the quality and dimensions depends on jigs, which are subject to wear.
Meanwhile, in order to solve these problems, graphite powder and thermoplastic or thermosetting binder resin powder are mixed in a dry state to manufacture a graphite composite, and the graphite composite is subjected to compression molding or injection molding using a press mold, thereby manufacturing a composite bipolar plate.
Conventional technologies have the above-described configuration, and generally use products manufactured according to a high-priced cutting method. In order to solve the problem of the product cost, the manufacture of the composite bipolar plate including the graphite and the resin is being studied using compression and injection processes.
A recently known composite bipolar plate obtained using compression molding or injection molding incurs considerably lower manufacturing cost than the conventional process using cutting and resin impregnation, but a further post-treatment process, such as surface polishing, needs to be performed due to the non-uniformity of product moldability. Therefore, the conventional technologies are limited in the ability to further reduce costs, and particularly, technology is required for selecting and dispersing a material suitable for compression and injection molding, which requires fluidity and dispersibility.
In addition, the above-described conventional technologies are based on a compact technology in which the size of a bipolar plate for fuel batteries is relatively small, for example, 100 cm2, but the use of the technologies for the purpose of a bipolar plate, having a size of about 700 cm2 or more, such as that of a redox flow battery, has not yet been achieved.
Further, in the case of a large-area bipolar plate, a defect rate of moldability is relatively very high, additional mold design and manufacturing costs are expected in order to maintain the uniform molding temperature and pressure of the mold, the surface of the product must be reprocessed, and moldability is poor for respective portions of the product due to the flow of a binder polymer into the surface of the product during compression molding.
In addition, the conventional technologies have problems in that a process for manufacturing the resin composite of carbon and a binder is very long and complicated, in that the intrinsic physical properties of the carbon and the polymer may be changed due to the manufacture of the composite at high temperatures and pressures, and in that expensive processing technology is required in order to uniformly control particle sizes.